1. Technical Field
The disclosed embodiments relate to Phase-Locked Loops (PLL) within local oscillators of receivers.
2. Background Information
A direct conversion receiver of a cellular telephone typically employs a mixer that mixes a Local Oscillator (LO) signal with a desired high frequency signal such that the desired high frequency signal is downconverted to a lower baseband frequency. A local oscillator circuit generates the LO signal. A local oscillator circuit typically involves employing a crystal oscillator circuit and a Phase-Locked Loop (PLL) circuit. The crystal oscillator circuit outputs a stable and fixed reference clock signal (XO). The PLL receives the signal XO and outputs the local oscillator signal LO that is in turn supplied to the mixer. The PLL allows the frequency of the LO signal to be changed so that the receiver can be tuned to downconvert a desired high frequency signal of interest. The receiver is tuned by changing the frequency of the LO signal.
Historically there have been two types of PLLs used in local oscillator circuits. One of the PLLs is referred to here as an “integer-N PLL”. The other of the PLLs is referred to here as a “fractional-N PLL”. FIG. 1 (Prior Art) is a simplified diagram of an integer-N PLL 1. A crystal oscillator 2 generates a very stable crystal oscillator output signal XO. The crystal oscillator may or may not be considered part of the phase-locked loop. The XO signal is frequency divided by a divider 3 to generate a very stable reference signal of fixed frequency referred to here as the “comparison reference clock signal” 4. The divisor by which divider 3 divides may, for example, have a different value depending on the band in which the receiver is to receive. A high frequency VCO output signal LO output by VCO 5 is divided down in frequency by a loop divider 6 to generate a divided down feedback signal 7. The feedback signal 7 is compared to the very stable comparison reference signal 4 by a phase detector 8. The error signal output by phase detector 8 passes through a charge pump 9 and a loop filter 10. Loop filter 10 supplies a current or voltage steering signal 11 to VCO 5 such that feedback signal 7 is phase-locked with respect to the comparison reference clock signal 4. The frequency of the LO signal can be changed by changing the integer divisor by which loop divider 6 frequency divides to LO signal to generate the feedback signal 7. The local oscillator signal LO generated by such an integer-N PLL generally exhibits a relatively large amount of phase noise. As the PLL operates, the frequency of the signal LO varies and is controlled within a frequency band determined by the bandwidth of loop filter 10.
As cellular telephones have come to be used for purposes other than just voice communication, cellular telephones are to be able to receive at higher and higher data rates. In order to increase data rates, it is generally true that phase noise of the LO must be reduced. It is therefore desired to use a PLL in the local oscillator circuit of the cellular telephone receiver that exhibits less phase noise than does the traditional integer-N PLL of FIG. 1.
FIG. 2 (Prior Art) is a diagram of a second type of PLL employed today in local oscillators of receivers of cellular telephones. This second type of PLL is referred to here as a “fractional-N” Phase-Locked Loop (PLL). Fractional-N PLL 12 involves a modulator 13 that changes the divisor by which the loop divider 14 divides. The divisor is changed such that over time the average frequency of the feedback signal 15 frequency and phase matches the frequency and phase of comparison reference clock signal 16. In a fractional-N PLL, the frequency of the comparison reference clock signal 16 can be higher, so there is no divider that divides down the frequency of the XO signal output by the crystal oscillator 17. Because a higher comparison reference clock signal frequency can be used, the loop filter can have a higher loop bandwidth. Increasing loop bandwidth typically suppresses phase noise. The fractional-N PLL topology therefore can be used to generate local oscillator signals that have less phase noise as compared to local oscillator signals that would be generated using the integer-N PLL topology.
Unfortunately, in some receiver applications, use of a fractional-N PLL has drawbacks as compared to use of an integer-N PLL. As a PLL operates, the steering signal supplied to the VCO changes as a function of the frequency of the comparison reference clock signal. This changing of the steering signal results in changes in the frequency of the LO signal. These changes evidence themselves in the frequency domain as harmonic frequency components around the center frequency of the LO signal. These harmonic frequency components are referred to as “spurs.”
FIG. 3 (Prior Art) is a diagram that illustrates an operational characteristic of the integer-N PLL 1 of FIG. 1. The Local Oscillator signal (LO) does not appear as a single ideal spike in the frequency domain but rather is pictured having skirts. The width of this skirt represents the phase noise that is present along with the LO signal itself. A desired high frequency signal is received on the antenna of the receiver and makes its way to the mixer of the receiver. The local oscillator signal LO supplied to the mixer is of such a frequency that the desired receive (RX) signal is downconverted in frequency to a baseband signal. Reference numerals 20-23 identify some of the spurs that are generated due to the steering of the VCO 5 in the integer-N PLL of FIG. 1. Note that the frequency separation between the spurs is FC1, the frequency of the comparison reference clock signal in the PLL of FIG. 1. Due to the relatively low frequency of the comparison reference clock signal in the integer-N PLL, the spurs are relatively close together and drop off in magnitude relatively rapidly such that there are effectively no spurs in the frequency channel 24. In the cellular telephone considered here, the transmitter of the cellular telephone may be transmitting at the same time that the receiver of the cellular telephone is receiving. The transmit frequency channel 24 is therefore separated in frequency from the frequencies of the desired RX signal. The double S symbols 25 in the diagram of FIG. 3 illustrate a large break in frequency. The frequency of the transmit channel is therefore separated in the frequency domain from the receive channel by a significant amount. As can be seen from the diagram of FIG. 3, the integer-N PLL generates an undesirable amount of phase noise.
FIG. 4 (Prior Art) is a diagram that illustrates an operational characteristic of the fractional-N PLL 12 of FIG. 2. Due to the greater loop bandwidth of the fractional-N PLL, the width of the skirt of the local oscillator signal LO in FIG. 4 is smaller than the width of the skirt of the local oscillator signal LO in FIG. 3. The fractional-N PLL exhibits less phase noise. Note, however, that the harmonic spurs components 26-28 are separated from one another in the frequency domain by the frequency FC2 of the comparison reference clock signal 16 in the fractional-N PLL 12 of FIG. 2. Frequency separation FC2 in the diagram of FIG. 4 is greater than frequency separation FC1 in the diagram of FIG. 3. Due to the greater frequency separation FC2 between harmonic spur components, the spurs of FIG. 4 do not drop off in magnitude as quickly as a function of frequency as do the spurs of FIG. 3. Such a spur 28 may therefore be of such a magnitude and such a frequency that it can reciprocally mix with transmitter leakage 24. Such reciprocal mixing may cause the mixer to downconvert the transmitter leakage to the baseband such that the downconverted transmitter leakage signal obscures the downconverted desired RX signal being received. This is undesirable. Use of the fractional-N PLL provided improved phase noise but unfortunately left the receiver susceptible to reciprocal mixing problems.